PON system

ABSTRACT

Disclosed herewith is a PON system and a bandwidth controlling method capable of controlling congestion with use of an upstream bandwidth in a PON section efficiently when congestion occurs in a gateway (GW) connected to an OLT. An OLT connected to a plurality of ONUs through a passive optical network (PON) and to a gateway (GW) through a communication line, when receiving a congestion occurrence notice indicating a congestion occurred output number from a GW, identifies the identifier of the ONU that is using a GW output line having the congestion output port number and shifts the bandwidth controlling of the PON section in a normal mode for allocating a bandwidth to each ONU normally to that in a bandwidth suppression mode for allocating a congestion time allowable bandwidth that is less than the current bandwidth to the ONU having the identified ONU identifier and a bandwidth to each of other ONUs according to its transmission queue length.

This application is a continuation application of U.S. Ser. No.11/702,155, filed Feb. 5, 2007, the entirety of which is incorporatedherein by reference.

CLAIM OF PRIORITY

The present application claims priority from Japanese application JP2006-107790 filed on Apr. 10, 2006, the content of which is herebyincorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to a passive optical network (PON) systemcomprising an optical line terminal (OLT) and plural optical networkunits (ONUs) connected to each other through a passive optical network(PON), more particularly to a method for controlling the bandwidth of aPON section when line congestion occurs in any of the output ports of aframe transfer device connected to the OLT.

BACKGROUND OF THE INVENTION

A carrier network for providing services of connection to the Internetconsists of an access network for holding a plurality of subscribersdirectly and a relay network (hereunder, to be referred to as a metronetwork) for bundling a plurality of access networks. And a passiveoptical network (PON) is one of such access network types.

A PON is a network system for connecting an OLT and a plurality of ONUsto each other through an optical fiber network provided with a functionfor branching an optical signal and enabling each ONU to transmit datato the OLT in a time division manner according to its access right (datatransmission time band) information notified from the OLT. The PONbranches an optical fiber connected to an OLT into a plurality of branchline optical fibers with use of an optical splitter (optical coupler) toenable an optical fiber section between an optical splitter and the OLTto be shared by a plurality of ONUs when an ONU disposed in the user'shouse is connected to a branch line optical fiber. Thus the PON canreduce the expenses for laying the optical fiber and hold many userterminals through the ONU.

The OLT of the PON system is connected to a metro network through, forexample, a gateway (GW). The GW is a frame transfer device for holding aplurality of OLTs and transferring each variable length frame betweeneach OLT and the metro network according to its header. In addition tothe GW for housing the OLT, the metro network also includes another GWfor housing, for example, an ISP network. Consequently, each userterminal connected to an ONU can access the Internet through the OLT,GW, and ISP network.

There are some types of PONs such as B-PON (Broadband PON) fortransmitting information with use of fixed length ATM cells in anoptical fiber section (PON section), G-PON (Gigabit PON) capable ofrealizing gigabit class high speed data transfer, and GE-PON(Giga-Ethernet PON) preferred to Ethernet (trade mark) services.

In the case of the G-PON capable of transferring variable length frames,an OLT generates GTC (G-PON Transmission Convergence) downstream framesspecific to a PON section and maps GEM (G-PON Encapsulation Mode) framesincluding packet data addressed to each ONU in the payload of each GTCdownstream frame. The GTC downstream frames are broadcast to a pluralityof branch line optical fibers through an optical splitter (opticalcoupler). Each ONU checks the ONU identification information (ONU portID) of the destination indicated by the header part of the GEM frameextracted from the GTC payload to receive the packet data addressed toitself selectively, then transfers the data to the subject target userterminal.

The PON OLT has a DBA (Dynamic Bandwidth Allocation) function forallocating a data transmission time band (time slot) for each ONUaccording to an amount of accumulated transmission data (transmissionqueue length) in each ONU. In the DBA, a time slot is allocated to oneor a plurality of ONUs in a predetermined cycle ΔT. Consequently, themaximum number of time slots to be allocated to one ONU becomes ΔT. If Bis assumed as a bandwidth of an optical fiber line, therefore, themaximum bandwidth B (limit) to be allocated to one ONU becomes ΔT×B. TheOLT DBA function distributes the maximum bandwidth B (limit) to beallocated in a cycle ΔT to a plurality of ONUs according to the amountof the data in the transmission queue in each ONU.

In the case of the G-PON, each time slot is specified with atransmission starting time and a transmission ending time. The OLT setsthe bandwidth control information of each ONU indicating both ONUidentification information and a time slot in the header part of a GTCdownstream frame, then notifies the information items to each ONU.Because the length varies among the branch line optical fiber sectionsin the PON, the transmission delay time of frames streaming from ONU toOLT also varies among ONUs. Consequently, the OLT measures thetransmission delay time of each ONU and notifies an equivalent delaytime to each ONU beforehand. Each ONU has a function for correcting thespecified transmission starting time with the equivalent delay time uponreceiving an allocated time slot from the OLT to start data transmissionat a proper timing.

The OLT transfers upstream frames received from the ONU to the GW. Inthis case, if the OLT transmission buffer is insufficient in capacity,the frames might be discarded. To avoid such frame discarding, forexample, JP-A No. 159203/2004 discloses a packet transfer device forlimiting the transmission data amount from every ONU housed in thesubject OLT by adjusting the access rights in a PON section in casewhere the accumulated data amount in the OLT built-in buffer exceeds apredetermined threshold value. In the description below, congestion isdefined as a state in which frames may be discarded if an amount ofaccumulated data that exceeds a predetermined threshold value is left asis.

In the field of communication networks, a back pressure (BP) techniqueis known well as a technique for preventing such discarding ofcommunication frames. According to the technique, congestion occurrenceis notified from a congestion-occurred node device to a datatransmission source device so that the data transmission source devicestops data transmission or adjusts the transmission data amount. Forexample, JP-A No. 153505/2004 discloses a data frame transmission systemthat controls congestion with use of a pose frame conforming to the IEEE(Institute of Electrical and Electronic Engineers) 802. 3. In JP-A No.153505/2004, if congestion occurs, the subject data frame transmissiondevice transmits a pose frame to the data transmission source device.Receiving the pose frame, the source device stops the data frametransmission or limits the bandwidth during a period specified with thepayload part of the pose frame.

SUMMARY OF THE INVENTION

In the case of a carrier network, a logic path set with such acommunication protocol as virtual local area network (VLAN) andmulti-protocol label switching (MPLS) is used for Internet connectionservices. Using such a logic path enables user frames received from aspecific subscriber terminal to be transferred along a predeterminedroute. If such a logic path is set, however, congestion might occur in agateway (GW) in which logic paths handling much data respectively areconcentrated as shown in FIG. 19 and some frames to be transferred mightbe discarded in the worst case.

In a gateway (GW) that houses plural PONs, even if a specific outputport goes into congestion, such frame discarding in the congestion portcan be avoided by applying a back pressure to each PON OLT. In otherwords, when such congestion occurs, a back pressure is applied to theOLT provided with a function for adjusting access rights in a PONsection as disclosed in JP-A No. 159203/2004, thereby the amount of dataflowing from OLT to GW is reduced. Furthermore, according to thetechnique disclosed in JP-A No. 153505/2004, the OLT can adjust accessrights in the PON section during a period specified in a pose frame.

However, because the GW that houses the OLT is provided with pluralinput/output ports (line interfaces), even when transmission data isconcentrated at a specific output port, the buffer of each of otheroutput ports is usually still sufficient in capacity. In the descriptionbelow, a “congestion port” is defined as a port in which the outputbuffer is insufficient in capacity due to transfer data concentration asshown in FIG. 19 and a “normal port” is defined as a port in which theoutput buffer is still sufficient in capacity even in such a congestioncase. And a logic path that passes such a congestion port is referred toas a “congestion port path” and a logic path that passes a normal portis referred to as a “normal port path”.

If a congestion control method disclosed in JP-A No. 159203/2004 isapplied in a case where a congestion port and a normal port coexist suchway, the amount of data transmitted from every ONU to which one of theOLTs connected to a GW is connected is suppressed. Consequently, evenwhen the congestion in the GW is eliminated due to the suppression ofthe data transmission from each OLT such way, even the bandwidth of thesubscriber (ONU) who uses a normal port path comes to be limited whilethe congestion is limited, thereby the throughput of the whole accessnetwork is lowered. That has been a problem.

Furthermore, according to the congestion controlling method disclosed inJP-A No. 159203/2004, each ONU allocated bandwidth is reduced while thecongestion is controlled without discriminating between the congestionport path user and the normal port user. Thus if an ONU transmissionbuffer overflows due to the limitation of the output bandwidth, thenormal port path user also comes to suffer from the frame discardingproblem.

Under such circumstances, it is an object of the present invention toprovide a PON system and a bandwidth controlling method capable ofcontrolling congestion by making good use of the upstream bandwidths ofa PON section when congestion occurs at a gateway (GW) connected to anOLT.

It is another object of the present invention to provide a PON systemand a bandwidth controlling method capable of controlling congestionwithout degrading the bandwidth allocated to each ONU that uses a GWnormal port when congestion occurs at a gateway (GW) connected to anOLT.

In order to achieve the above objects, a PON system of the presentinvention is connected to plural optical network units (ONU) through apassive optical network (PON) and an optical line terminal (OLT)connected to a gateway (GW) through a communication line, when receivinga congestion occurrence notice indicating a number of the output port inwhich congestion occurred (congestion occurred output port number) fromthe GW, identifies the ID of the ONU that uses a GW output line havingthe congestion output port number and shifts the PON section bandwidthcontrolling in a normal mode for allocating a bandwidth to each ONU tothat in a bandwidth suppressing mode for allocating a congestion timeallowable bandwidth that is less than the current bandwidth to an ONUhaving the identified ONU identifier and allocating a bandwidth to eachof other ONUs according to its transmission queue length.

More concretely, in the PON system of the present invention, the OLTincludes a congestion control table that records a relationship betweena GW output port number and the identifier of each ONU that uses a GWoutput line identified with the output port number; an OLT controllingpart that records a transmission queue length notified in an upstreamPON frame from each ONU beforehand and allocates a bandwidth distributedto each of the ONUs dynamically as the next upstream frame transmissionbandwidth in a PON section according to its transmission queue length;and a downstream frame generation part that generates a downstream PONframe including bandwidth control information for indicating an upstreamframe transmission bandwidth of each ONU according to the frametransmission bandwidth allocated by the OLT controlling part. Whenreceiving a congestion occurrence notice that indicates a congestionoccurred output port number from the GW, the OLT control part identifiesthe ONU ID corresponding to the congestion occurred output port numberin the bandwidth control table and shifts the PON section bandwidthcontrol in a normal mode for allocating a bandwidth to that in abandwidth suppressing mode for allocating a congestion time allowablebandwidth that is less than the current bandwidth to an ONU having theidentified ONU ID and allocating a bandwidth to each of other ONUsaccording to its transmission queue length.

In the first embodiment of the present invention, the OLT control partincludes a bandwidth control table for recording a transmission queuelength notified from an ONU, an allocated bandwidth calculated in apredetermined cycle according to the transmission queue length, and acongestion flag corresponding to the ONU ID respectively. When receivinga congestion occurrence notice from the GW, the OLT control part changesthe state of the congestion flag corresponding to the ONU ID identifiedin the bandwidth control table to a congestion display state, thenallocates a bandwidth to each ONU of which congestion flag indicates thenormal state according to its transmission queue length and allocates acongestion time allowable bandwidth to each ONU of which congestion flagindicates the congestion display state during the bandwidth controllingin the bandwidth suppression mode.

Furthermore, in the first embodiment of the present invention, thedownstream frame generation part generates bandwidth control informationof each ONU according to its congestion flag state set in the bandwidthcontrol table and, in the downstream PON frame, notifies each ONU ofwhich congestion flag indicates the normal state in the downstream PONframe of a bandwidth according to its transmission queue length andnotifies each ONU of which congestion flag indicates the congestiondisplay state of a congestion time allowable bandwidth.

The OLT control part, when receiving a congestion reset noticeindicating a number of the output port in which congestion is reset(congestion reset output port number) from the GW, identifies the ONU IDcorresponding to the congestion reset output port number with referenceto the bandwidth control table beforehand and changes the state of thecongestion flag corresponding to the ONU ID identified in the bandwidthcontrol table to the normal state at a predetermined timing.

According to the present invention, when congestion occurs at a specificoutput port of a GW to which an OLT is connected as shown in FIG. 20,the OLT can delete the bandwidth of the specific ONU that istransmitting communication frames to the congestion occurred output portselectively, thereby the OLT can distribute a surplus bandwidth in thePON section generated due to the deletion to other ONUs. Thus the GWcongestion state can be eliminated without lowering the whole systemthroughput.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for showing a configuration of a network to whichthe present invention applies;

FIG. 2 is an example of a downstream frame format in a PON section;

FIG. 3 is an example of a format of a congestion occurrence/resetnotification frame to be transmitted from a GW;

FIG. 4 is a block diagram of a configuration of an OLT;

FIG. 5 is an example of a bandwidth control table provided for an OLT;

FIG. 6 is an example of a congestion control table provided for an OLT;

FIG. 7 is a block diagram of a configuration of an ONU;

FIG. 8 is a block diagram of a configuration of a GW;

FIG. 9 is a flowchart of an example of bandwidth control processingsexecuted by an OLT;

FIG. 10 is an explanatory diagram for describing bandwidth allocation byan OLT in a normal mode and in a bandwidth suppression mode;

FIG. 11 is a flowchart of downstream PON frame generation processingsexecuted by an OLT;

FIG. 12 is a detailed flowchart of an example of processings forgenerating a bandwidth information field shown in FIG. 11;

FIG. 13 is an example of a congestion control sequence executed amongGW, OLT, and ONU;

FIG. 14 is a diagram for describing a relationship between a change of aGW output frame buffer queue length and bandwidth controlling by an OLT;

FIG. 15 is an explanatory graph for describing temporal changes of atransmission queue length and an allocated bandwidth in an ONU that istransmitting a frame that passes a GW congestion port;

FIG. 16 is an explanatory graph for describing temporal changes of atransmission queue length and an allocated bandwidth in an ONU that istransmitting a frame that passes a normal port of a GW;

FIG. 17 is a configuration of a communication network to which thepresent invention applies in another embodiment;

FIG. 18 is a block diagram of a configuration of a GW 500-1 shown inFIG. 17;

FIG. 19 is an explanatory diagram for describing a place where a frameis to be discarded in a GW and how a bandwidth is used in each placeaccording to an employed conventional bandwidth controlling method; and

FIG. 20 is an explanatory diagram for describing a place where a frameis to be discarded and how a bandwidth is used in each place in a GW towhich the present invention applies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereunder, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a diagram for showing an example of a communication network towhich the present invention applies.

A carrier network consists of an access network and a metro network. Theaccess network includes optical line terminals (OLT) 10 (10-1-1 to10-2-n) and plural optical network units (ONU) 30 (30-1 to 30-n)connected to each other through a passive optical network (PON)respectively. The metro network includes plural gateways (GW) 50 (50-1to 50-4) connected to each other through lines 101 (101-1 to 101-4).

Each of subscriber terminals TE (TE-1-1 to TE-n-m) is connected to theONU 30 through one of the lines 109 (109-1 to 109-n-m) and connected tothe metro network through a PON and an OLT 10. The gateways GW 50 of themetro network are divided into two types; GWs like 50-1 and 50-2connected to an OLT 10 through the lines 105 (105-1-1 to 105-2-n) andGWs like 50-3 and 50-4 connected to ISP networks 103 (103-1 and 103-2)through the lines 102 (102-1 and 102-2). Each IPS network 103 isprovided with ISP servers 90 (90-1 and 90-2) and each subscriberterminal TE accesses the Internet 114 through an ISP server 90. On eachof connection lines 101, 102, 105, and 109 are transferred communicationframes according to the Ethernet (trade mark) protocol.

The passive optical network PON consists of optical fibers 106 (106-1 to106-n) housed in an OLT 10, branch line optical fibers 108 (108-1-1 to108-n-m) connected to each ONU 30, and optical splitters 107 (107-1 to107-n). The PON is structured, for example, as a G-PON conforming to theITU-T Recommendation G. 984. 1.

A downstream optical signal transmitted from an OLT 10 through anoptical fiber 106 is branched by an optical splitter 107 into pluralbranch line optical fibers 108 and broadcast to plural ONUs 30 connectedto those branch optical fibers 108. On the contrary, upstream opticalsignals transmitted from each ONU 30 through branch line optical fibers108 are multiplexed by an optical splitter 107 and transferred to an OLT10 through an optical fiber 106. In this embodiment, it is assumed thatboth upstream and downstream optical signals are subjected to afrequency multiplexing process and transmitted through the same opticalfiber. However, it is also possible to divide the optical fibers 106 and108 in a PON section into an upstream one and a downstream one, therebythe same wavelength optical signal is applied to the upstream anddownstream optical signals. Although the metro network connects pluralgateways GW 50 to each other in a ring pattern with the line 101 in FIG.1, the metro network may also connects those gateways GW 50 in anotherpattern, for example, in a mesh pattern.

The present invention premises that a fixed data path is provided foreach ONU 30 in the metro network. For example, assume now that a userconnected to the ONU 30-1 has made a contract with a provider of the ISPnetwork 103-1 for a connection service to the Internet. In this case,the GW 50-1 always transfers communication frames received from the ONU30-1 through the OLT 10-1-1 to the connection line 101-2 of the ISPnetwork 103-1. At first, the ISP server 90-1 of the ISP network 103-1executes predetermined communication procedures such as userauthentication, IP address allocation, etc. with respect to eachsubscriber terminal TE, then enables the communication between thesubscriber terminal TE and the Internet 104.

In the same way, if the user connected to the ONU 30-n has made acontract with a provider of the ISP network 103-1 for a connectionservice to the Internet, the GW 50-1 always comes to transfer framesreceived from the ONU 30-n through the OLT 10-1-n to the connection line101-2.

According to the present invention, each OLT 10 already knows a GWoutput port through which frames from each ONU 30 connected through thePON are transmitted. If congestion occurs in a GW transmission queue,therefore, the OLT 10 controls the bandwidth in the PON section bymaking good use of the relationship between the ONU 30 and the GW.

FIG. 2 is an example of a format of downstream frames in a PON section,transmitted from the OLT 10 to the ONU 30. Here, a GTC downstream frameapplied to the G-PON is shown.

A GTC downstream frame 60 consists of a header part (PCBd) 61 and a GTCpayload 62. The GTC payload 62 includes plural unicasting framesaddressed to a specific ONU and broadcasting frames or multicastingframes to be received by plural ONUs in the GEM frame format specific toa PON section. Each ONU checks the ONU ID set in the header of the GEMframe extracted from the GTC payload 62 to select each frame itreceives.

The OLT 10 transmits each GTC downstream frame 60 in the basic framecycle ΔTF, for example, in cycles of 125 microseconds. The header part61 of the GTC downstream frame consists of a Psync field 611 thatincludes a specific signal pattern to enable the receiving side (ONU 30)to synchronize with frames, a bandwidth information field 613 fornotifying each ONU 103 of an upstream frame transmission time band, anONU control information field 614 for notifying each ONU of such controlinformation as starting, stopping, etc., and a field of other fieldinformation 612. The frame information field 612 includes, for example,such information as whether to make an FEC processing in the subjectframe, a frame counter, a BIP, a header length, etc. The contents in theframe information field 612 and the ONU control information field 614are deleted in some cases according to the circumstances.

The bandwidth information field 613 consists of plural bandwidth controlinformation fields 613-1, 613-2 . . . provided for each ONU. Eachbandwidth control information field 613-i indicates a bandwidth controlID (Alloc-ID) 621 for identifying an ONU, a transmission starting time622 and transmission ending time 623 for indicating an upstream frametransmission time band (allocated bandwidth), and other controlinformation 624. The other control information 624 includes, forexample, information about whether to notify the subject transmissionqueue state from ONU to OLT and control information for specifyingwhether to require an FEC in the subject upstream frame.

The destination ONU (Alloc-ID) 621 of the bandwidth information notifiedin the bandwidth information field 613 has no relation to thedestination ONU in the GEM frame included in the payload 62. The OLT 10calculates a bandwidth to be allocated to each ONU 30 under its controlin cycles of 125 microseconds×N (e.g., N=4 to 6). The OLT 10 divides theupstream transmission time band of 125 microseconds×N into plural timebands so as to allocate a longer transmission time band to each ONUhaving a lot of data in the transmission queue according to the state ofthe upstream transmission queue in each ONU (e.g., a queue lengthindicating an amount of data to be transmitted) to determine a bandwidthto be allocated to each ONU.

For example, if m units of ONUs 30 are connected to an OLT 10, each GTCdownstream frame includes bandwidth control information related to someof the m units of ONUs. The OLT 10 completes notice of an allocatedbandwidth to each ONU by transmitting GTC downstream frames by N timesconsecutively. Each ONU finds bandwidth control information having theAlloc-ID of itself in a received GTC downstream frame to transmitupstream data according to the transmission starting time 622 and thetransmission ending time 623 indicated by the bandwidth controlinformation.

According to the present invention, the GW 50 monitors the transmissionqueue length QL at each output port and when the transmission queuelength QL exceeds a predetermined first threshold HT1, the GW 50transmits a congestion occurrence notification frame that specifies anoutput port number to the OLT 10 to prevent communication framediscarding to be caused by insufficient capacity of the buffer. Each OLT10 allocates a bandwidth to each OLT in the bandwidth suppression modefor suppressing only the amount of transmission data from a specific ONUto be transmitted to the output port specified in the above congestionoccurrence notification frame (hereunder, to be referred to as acongestion port).

When the congestion port transmission queue length falls below thesecond threshold TH2 that is lower than the first threshold TH1 due to aback pressure to the OLT, the GW 50 transmits a congestion resetnotification frame that specifies the subject output port number to eachOLT 10. Each OLT 10 then resets the suppression of the amount oftransmission data to be transmitted to the output port specified in thecongestion reset notification frame and returns to the bandwidthallocation control in the normal mode according to the state of theupstream transmission queue in each ONU.

In the case of the network configuration shown in FIG. 1, for example,if the length of the transmission queue of an output port connected tothe line 101-2 exceeds the first threshold value TH1, the GW 50-1transmits a congestion occurrence notification frame to each of the OLTs10-1-1 to 10-1-n, thereby each of the OLTs 10-1-1 to 10-1-n reduces thebandwidth allocated to the ONU that is communicating with the Internet104 through an ISP network 103.

As to be described later, in the first embodiment of the presentinvention, each OLT is provided with a congestion control table forrecording a relationship between each output port number and each ONU IDof the connected GW 50. Thus the OLT, when receiving a congestionoccurrence notification frame from the GW 50, identifies the ONU IDcorresponding to the congestion port number with reference to thecongestion control table and allocates a bandwidth to each OLT in thebandwidth suppression mode so as to reduce the allocated bandwidth tothe ONU having this ONU ID.

Also in the first embodiment of the present invention, each OLT isprovided with a bandwidth control table for indicating the relationshipamong a transmission queue length, the current allocated bandwidth, anda predetermined congestion time allowable bandwidth with respect to anONU ID respectively. When an ONU ID corresponding to a congestion portnumber is identified, therefore, each OLT reduces the bandwidthallocated to the ONU having this ONU ID up to a congestion timeallowable bandwidth specified in the bandwidth control table. In thebandwidth suppression mode, because the bandwidth in the PON sectioncomes to have a surplus due to the reduction of the amount of datatransmitted to the congestion port from the ONU, the OLT 10 uses the PONsection bandwidth effectively by increasing the bandwidth allocated toanother ONU that is transmitting data to a normal port.

FIG. 3 is an example of a format of the congestion occurrence/resetnotification frame (back pressure control frame) transmitted from GW 50to OLT 10.

The back pressure control frame uses, for example, a frame formatprovided with an Ethernet VLAN (Virtual Local Area Network) tag. Theback pressure control frame includes a destination MAC (Media AccessControl) address 601, a source MAC address 602, a VLAN tag 603, acongestion bit 604, and a congestion port number 605.

The destination MAC address 601 indicates the MAC address of each OLT 10housed in a GW 50 and the source MAC address 602 indicates the MACaddress of a network interface that houses a connection line between theGW 50 and OLT 10. The VLAN tag 603 has a specific value set to indicatethat the subject frame is a back pressure control frame. The congestionbit 604 has the value “1” for a congestion occurrence notification frameand “0” for a congestion reset notification frame. The congestion portnumber 605 indicates a congestion detected GW output port number.

Receiving a back pressure control frame, each OLT 10 can recognize thereceived frame as a back pressure control frame according to the valueset in the VLAN tag 603 and determine the received frame as a congestionoccurrence notification or congestion reset notice according to thevalue set in the congestion bit 604. Each OLT 10 can also identify anONU of which allocated bandwidth is to be reduced with reference to thecongestion port number 605, the congestion control table, and thebandwidth control table.

FIG. 4 is a block diagram of a configuration of an OLT 10.

The OLT 10 consists of an OLT control part 11, a parameter memory 12, anoptical transmission/receiving part 13 connected to a PON optical fiber106, an Ethernet interface (IF) 14 connected to an Ethernet line 105 forconnecting a GW 50, as well as an upstream signal processing circuit anda downstream signal processing circuit connected respectively betweenthe optical transmission/receiving part 13 and the Ethernet interface IF14. The parameter memory 12 includes a bandwidth control table 120 to bedescribed in FIGS. 5 and 6 and a congestion control table 130 formedrespectively in itself.

The upstream signal processing circuit consists of an optical/electricalconversion part 15 for converting each optical signal received by theoptical transmission/receiving part 13 to an electrical signal, anupstream frame terminal part 16 connected to the optical/electricalconversion part 15, an upstream data buffer 17 for temporarily storingupstream frames output from the upstream frame terminal part 15, and anupstream frame transmission part 18 for reading frames from the upstreamdata buffer 17 and transmitting those frames to the Ethernet IF 14. Theupstream frame terminal part 16 regenerates an upstream frame from eachoutput signal of the optical/electrical conversion part 15 and outputssuch control information as a source queue length extracted from theframe header to the OLT control part 11, as well as converts eachreceived frame to a data frame in the Ethernet format and outputs thedata frame to the upstream data buffer 304.

The downstream signal processing circuit consists of a frame analysispart 19 connected to the Ethernet IF 14, a downstream data buffer 20 fortemporarily storing user frames output from the data analysis part 19, aframe generation part 21 for generating a downstream PON frame (GTCdownstream frame) described in FIG. 2 in a predetermined cycle ΔTF andmapping each user frame read from the downstream data buffer 20 orcontrol frame output from the OLT control part 11 in the GEM frameformat, a downstream frame transmission part 22 for transmitting eachdownstream frame output from the downstream frame generation part 21 asan electrical signal, and an electrical/optical conversion part 23 forconverting each electrical signal output from the downstream frametransmission part 22 to an optical signal. The frame analysis part 19analyzes frames received from the Ethernet IF 14 and outputs user framesto the downstream data buffer 20 and back pressure control frames to theOLT control part 11 respectively.

The OLT control part 11 calculates a bandwidth to be allocated to eachONU 30 in a predetermined cycle according to the source queue lengthindicated by the bandwidth control table 120 formed in the parametermemory 12 and updates the bandwidth control table 120 with the result.How the OLT control part 11 calculates a bandwidth to be allocated suchway will be described more in detail later with reference to FIG. 9.

As shown in FIG. 5, the bandwidth control table 120 consists of pluraltable entries, each corresponding to a bandwidth control ID (Alloc-ID)121 that is an ONU ID. Each table entry indicates a congestion timeallowable bandwidth (Bcon) 122 predetermined for each ONU, a maximumcontrol bandwidth (Bmax) 123, an allocation bandwidth (BW) 124calculated by the OLT control part 11, an ONU source queue length (SQ)125, a congestion flag 126, and a carry-over bandwidth 127. The sourcequeue length (SQ) 125 indicates the latest value of the transmissionqueue of the ONU 30 notified from the upstream frame terminal part 303.

As shown in FIG. 6, the congestion control table 130 consists of acongestion flag 132 corresponding to a GW output port number 131 andplural table entries, each indicating a relationship with an ONU ID(bandwidth control ID) 133 that uses a GW output line identified withthe GW output port number 131.

The OLT control part 11, when receiving a back pressure control frame asshown in FIG. 3 from the frame analysis part 19, refers to thecongestion control table 130 formed in the parameter memory 12 to updatethe congestion flag 132 of a table entry corresponding to the congestionport number 605 indicated in the back pressure control frame accordingto the congestion bit 605 in the received frame, then identifies the ID133 of the ONU in which the congestion is to be controlled. After that,the OLT control part 11 searches for a table entry corresponding to thebandwidth control ID 133 in the bandwidth control table 120 and updatesthe value of the congestion flag 126. If the back pressure control frameis a congestion occurrence notification frame (congestion flag=“1”), thevalue of the congestion flag 126 is changed to “1” immediately. If theback pressure control frame is a congestion reset notification frame(congestion flag=“0”), the value of the congestion flag 126 is changedto “0” at the next bandwidth calculation time.

FIG. 7 is a block diagram of a configuration of an ONU 30.

The ONU 30 consists of an ONU control part 31, an Ethernet interface(IF) 32 connected to an Ethernet line 109 for connecting a subscriberterminal TE, an optical transmission/receiving part 33 connected to aPON branch line optical fiber 108, as well as an upstream signalprocessing circuit and a downstream signal processing circuit connectedrespectively between the Ethernet interface IF 32 and the opticaltransmission/receiving part 33.

The ONU control part 31 consists of an upstream data buffer managementpart 311, a control frame generation part 312, and an upstreamtransmission timing generation part 313.

The upstream signal processing circuit consists of an upstream databuffer 34 for temporarily storing upstream user frames output throughthe Ethernet IF, an upstream frame generation part 35 for generatingupstream PON frames, an upstream frame transmission part 36 fortransmitting upstream PON frames generated by the upstream framegeneration part 35 in a time band indicated by the upstream transmissiontiming generation part 313, and an electrical/optical conversion part 37for converting each electrical signal output from the upstream frametransmission part 36 to an optical signal.

The downstream signal processing circuit consists of anoptical/electrical conversion part 38 for converting each optical signalreceived by the optical transmission/receiving part 33 through a branchline optical fiber 108 to an electrical signal, a downstream frameterminal part 39 connected to the optical/electrical conversion part 38,a downstream data buffer 40 for temporarily storing user framesaddressed to the ONU of itself from the downstream frame terminal part39, and a downstream frame transmission part 41 for transmitting userframes read from the downstream data buffer 40 and control framessupplied from the control frame generation part 312 to the Ethernet IF32 respectively.

The downstream frame terminal part 39 converts each signal output fromthe optical/electrical conversion part 38 to a downstream PON frame (GTCdownstream frame) and supplies information of an allocated bandwidth tothe self-ONU, extracted from the frame header (PCBd), to the upstreamsignal timing generation part 313. The downstream frame terminal part 39also extracts each user frame addressed to the self ONU from the GTCpayload and converts the user frame to a data frame in the Ethernetformat and outputs the frame to the downstream data buffer 40.

The upstream transmission timing generation part 313 determines anupstream frame transmission time band according to the transmissionstarting time and the transmission ending time indicated by theallocated bandwidth information, as well as according to an equivalentdelay time notified beforehand from the OLT 10, then controls theupstream frame generation part 35, the upstream frame transmission part36, and the upstream buffer management part 311 respectively.

The upstream data buffer management part 311 monitors the amount of data(transmission queue length) stored in the upstream data buffer 34. Whenthe transmission queue length exceeds a predetermined threshold value,the management part 311 issues a control signal to the control framegeneration part 312, which then generates a pose frame and notifies theupstream frame generation part 35 of the current transmission queuelength at a timing specified by the upstream transmission timinggeneration part 313. The upstream frame generation part 35 includesinformation of a transmission queue length notified from the upstreambuffer management part 311 in the header according to a command from theupstream transmission timing generation part 313 and generates anupstream PON frame including a user frame read from the upstream databuffer 34 in the payload, then outputs the PON frame to the upstreamframe transmission part 36.

The control frame generation part 312 generates a control frame (poseframe) used to instruct an object subscriber terminal TE to suppressdata transmission according to a control signal received from theupstream data buffer management part 311, then outputs the control frameto the downstream frame transmission part 41. The downstream frametransmission part 41 transmits each user frame read from the downstreamdata buffer 40 to the Ethernet IF 32 if there is no pose frame generatedfrom the control frame generation part 312. If a pose frame isgenerated, the control frame generation part 312 transmits the poseframe to the Ethernet IF 32.

After suppressing data transmission in response to the pose frame, ifthe subscriber terminal TE has a queue length of the upstream databuffer 34, which falls below the predetermined congestion resetthreshold value, the upstream data buffer management part 311 issues acontrol signal for resetting the congestion to the control framegeneration part 312. In response to the control signal, the controlframe generation part 312 generates a control frame to reset thesuppression of the data transmission and outputs the control frame tothe downstream transmission part 41.

FIG. 8 is a block diagram of a configuration of a GW 50-1.

The GW 50-1 consists of plural network interface (NIF) parts 51 (51-1 to51-q) connected to an Ethernet line (105 or 101) respectively, a framerelay part 52 for connecting those NIF parts 51 to each other, a GWmanagement part 53, and a management information table 54 for holding GWmanagement information to be referred to from the GW management part 53.Each NIF part 51 consists of an input line interface part 51A connectedto the Ethernet IF 511 for housing Ethernet input/output lines and anoutput line interface part 51B.

The input line interface part 51A consists of a parameter table 513 forholding parameter information used for header processing, an inputheader analysis part 512 for analyzing the header of each frame receivedfrom the Ethernet IF 511 and adding an internal header includinginternal routine information to the received frame according to theparameter table 513, an input frame buffer 514 for temporarily storingframes output from the input header analysis part 512, and an inputframe reading part 515 for reading frames from the input frame buffer514 and outputting the read frames to the frame relay part 52.

The frame relay part 52 relays each frame received from each input lineinterface part 51A to the output line interface part 51B of a specificoutput port determined by the internal routine information. As describedabove, according to the present invention, because a fixed path is setfor each ONU in a carrier network, the frame relay part 52 comes totransfer frames through a fixed output port in each ONU 103.

The output line interface part 51B consists of a parameter table 517, anoutput header analysis part 516 for analyzing each frame received fromthe frame relay part 52 to remove the internal header and make a headerprocessing according to the data set in the parameter table 517, anoutput frame buffer 518 for temporarily storing frames output from theoutput header analysis part 516, a queue length monitor 519 formonitoring the amount of data (queue length) stored in the output framebuffer 519 and outputting queue length information to the GW managementpart 53, an output frame reading part 520 for reading frames output fromthe output frame buffer 518 and outputting the frames to the Ethernet IF511, and a back pressure control frame generation part 521.

The parameter table 517 records parameter information required for aheader processing, a destination MAC address (OLT MAC address) requiredto generate a back pressure control frame, and a source MAC address(Ethernet IF MAC address). However, the output line interface part 51Bto which no OLT is connected is not required to record those MACaddresses. The management information table 54 records the firstthreshold value TH1 for detecting congestion occurrence, the secondthreshold value TH2 for detecting congestion reset, and a congestionflag indicating a congestion state in each output port (output lineinterface part 51B). The first and second threshold values are in arelationship of TH1>TH2.

The GW management part 53, when receiving a queue length QL from thequeue length monitor 519 of each output line interface part 51B,compares the queue length QL with the threshold values TH1 and TH2. Ifthe queue length QL of the output frame buffer 518 of an output port Piexceeds the first threshold value TH1, the GW management part 53 sets“1” in the congestion flag corresponding to the output port Pi in themanagement information table 54 and notifies the congestion occurrenceto the back pressure control frame generation part 521 belonging to eachof other output ports.

As for the output port Pi of which congestion flag is set at “1”, the GWmanagement part 53 compares the queue length QL of the output framebuffer 518 to be received later from the queue length monitor 519 withthe second threshold value TH2. When the queue length QL is under thesecond threshold value TH2, the GW management part 53 changes the valueof the congestion flag to “0” and notifies the back pressure controlframe generation part 521 belonging to each of other output ports, ofcongestion reset. The congestion occurrence notice and the congestionreset notice respectively include a VLAN tag value, a congestion bitindicating occurrence/reset of congestion, an output port numberindicating a congestion occurred (reset) output line interface part 51B.

The back pressure control frame generation part 521 of each output lineinterface part 51B, when receiving a congestion occurrence or resetnotice from the GW management part 53, reads the object MAC addressinformation from the parameter table 517 to generate a back pressurecontrol frame shown in FIG. 3 and output the frame to the output framereading part 520. The output frame reading part 520, when receiving theback pressure control frame and completing the transmission of one ofthe frames being read from the output frame buffer 518, transmits thecontrol frame to the back pressure control frame Ethernet IF 511, thenrestarts reading/transmission of frames from the output frame buffer 518later.

FIG. 9 is a flowchart of bandwidth control processings executed by theOLT control part 11 of an OLT 10. The OLT control part 11 executes abandwidth control processing 110 in the basic frame cycle ΔTF (in cyclesof 125 microseconds) shown in FIG. 2 to calculate a bandwidth to beallocated to each ONU in the ΔTF×N cycle. In the description below, theN value is represented as MAX and the number of execution times of thebandwidth control processing 110 is represented by a parameter i.

In the bandwidth control processing 110, the OLT control part 11increases the value of the parameter i (i=i+1) (step 111) and checkswhether or not a new back pressure control frame is received from the GW(step 112). If no back pressure control frame is received, the OLTcontrol part 11 determines whether or not the parameter i value hasreached MAX (step 117). If the determination result is NO (>MAX), theOLT control part 11 exits the processing. If the determination result isYES (=MAX), the OLT control part 11 calculates a bandwidth to beallocated (to be described later) and updates the bandwidth controltable 120.

If a new back pressure control frame is received from the GW, the OLTcontrol part 11 checks the congestion bit 604 included in the receivedframe (step 113). If “1” is set in the congestion bit 604, that is, ifthe received frame is a congestion occurrence notification frame, theOLT control part 11 sets “1” in the congestion flags 132 and 126corresponding to the congestion port number 605 indicated in thereceived frame in the congestion control table 130 and bandwidth controltable 120 (step 114), then determines whether or not the value of theparameter i has reached the MAX (step 117).

Concretely, in step 114, the OLT control part 11 searches the subjecttable entry corresponding to the congestion port number 605 in thecongestion control table 130 and sets “1” in the congestion flag 132,then refers to the bandwidth control table 120 according to thebandwidth control ID 133 indicated by the table entry to set “1” in thecongestion flag 126 of each table entry corresponding to the bandwidthcontrol ID 133.

In case where “0” is set in the congestion bit included in the receivedframe, that is, if the received frame is a congestion reset notificationframe, the OLT control part 11 records the value of the congestion portnumber 605 in the work area as a congestion reset port number (step116), then determines whether or not the parameter i value has reachedthe MAX (step 117). If the determination result is NO (<MAX), the OLTcontrol part 11 exits the processing.

If the parameter i value has reached the MAX, the OLT control part 11checks the congestion reset port number in the work area (step 118). Ifno congestion reset port number is recorded in the area, the OLT controlpart 11 calculates a new bandwidth to be allocated to each ONU withreference to the bandwidth control table 120 and records the newallocation bandwidth 124 in the bandwidth control table 120 (step 121).After that, the OLT control part 11 sets the initial value “1” in theparameter i and exits the processing.

If a congestion reset port number is recorded in the work area, the OLTcontrol part 11 sets “0” in the congestion flags 132 and 126corresponding to the congestion reset port number in the congestioncontrol table 130 and in the bandwidth control table 120 respectively(step 119), then clears the congestion reset port number in the workarea (step 120). After that, the OLT control part 11 calculates a newbandwidth to be allocated to each ONU (step 121).

According to the present invention, the bandwidth allocation calculationmode executed by the OLT control part 11 includes a normal mode and abandwidth suppression mode. In the normal mode, for example, a bandwidthis allocated to each ONU according to a source queue length 125indicated in the bandwidth control table 120 with use of a dynamicbandwidth allocation (DBA) function ruled by the ITU-T RecommendationsG983.4 and G984.3. Concretely, for example, the OLT control part 11finds a total value of the source queue lengths 125 in the bandwidthcontrol table 120 and calculates a weight of each ONU according to theratio between the total queue length value and each ONU source queuelength 125, then distributes an upstream bandwidth to each ONU in theΔTF×N period of the PON section.

In the bandwidth suppression mode, a bandwidth is allocated only whencongestion occurs in any of the output ports of a GW. In the bandwidthsuppression mode, the OLT control part 11 allocates a bandwidthindicated by the congestion time allowable bandwidth 122 to an ONU forwhich “1” is set in the congestion flag 126 in the bandwidth controltable 120. Because the congestion time allowable bandwidth 122 issmaller than the bandwidth 124 allocated in the normal mode, a surplusis generated in the upstream bandwidth of the PON section if a logicalpath communication bandwidth going to a congestion port of a GW isreduced to a congestion time allowable bandwidth 122 from an allocatedbandwidth 124. In the bandwidth suppression mode of the presentinvention, therefore, the OLT control part 11 reduces the total value ofthe congestion time allowable bandwidths 122 indicated by each tableentry of which congestion flag 126 is set at “1” from the upstreambandwidth in the ΔTF×N period of the PON section and distributes theremaining bandwidth to each of other ONUs just like in the normal mode.

FIG. 10 is an explanatory diagram for describing how an OLT 10 allocatesa bandwidth in the normal mode and in the bandwidth suppression moderespectively.

To simplify the description, it is assumed here that 8 units of ONUs 30are connected to an OLT 10 and the OLT control part 11 calculates abandwidth to be allocated to each ONU in cycles of ΔTD, which is threetimes the basic frame cycle ΔTF (N=3). And also to make it easier tounderstand the difference between the bandwidth allocation in the normalmode and that in the bandwidth suppression mode, it is assumed here thateach ONU transmission queue length is fixed in each period shown in FIG.10. Each of slash-marked frames F2 (F12, F22), F3 (F13, F23), and F6(F16, F26) represents a bandwidth allocated to the ONU 2, ONU 3 and anONU 6 that are forwarding to a GW congestion port.

In this embodiment, the OLT control part 11 updates the value of theallocated bandwidth 124 in the bandwidth control table 120 in cycles ofΔTD and the downstream frame generation part 21 notifies each ONU of theallocated bandwidth 124 indicated in the bandwidth control table 120.The OLT control part 11, when receiving a congestion notice from the GW50, sets “1” in the congestion flag 126 corresponding to the congestionport in the bandwidth control table 120. In this embodiment, thedownstream frame generation part 21 checks the congestion flag 126 inthe bandwidth control table 120 and notifies each ONU for which “0” isset in the congestion flag 126 of the allocated bandwidth 124 andnotifies each ONU for which “1” is set in the congestion flag 126 of thecongestion time allowable bandwidth 122.

The frame cycles ΔTF(1) to ΔTF(3) shown in (A) of FIG. 10 indicate theupstream frame transmission bandwidths of the ONU 1 to an ONU 8allocated in the normal mode. (B) indicates an upstream frametransmission bandwidth in frame cycles ΔTF (4) to ΔTF(6) when the OLTcontrol part 11 receives a congestion notice from the GW 50 while theONU 2 is transmitting a frame F12.

As described above, because the OLT control part 11 updates the value ofthe allocated bandwidth 124 in the bandwidth control table 120 in cyclesof ΔTD, the value of the allocated bandwidth 124 of each ONU is notchanged in frame cycles of ΔTF(4) to ΔTF(6). In this embodiment,however, the downstream frame generation part 21 notifies each ONU forwhich “1” is set in the congestion flag 126 of the congestion timeallowable bandwidth 122. Consequently, the bandwidth of the upstreamframes streaming from the ONU 6 to a GW congestion port is reduced,thereby an empty bandwidth BW (V) is generated at the end of the framecycle TF(6).

If the OLT control part 11 updates the value of the allocated bandwidth124 in the bandwidth control table 120 between frame cycles ΔTF(6) andΔTF(7), the bandwidths to be notified to ONU 1 to ONU 8 are changed inframe cycles ΔTF(6) TF(7) to ΔTF(6) TF(9) as shown in (C) of FIG. 10.Because of the bandwidth allocation in the bandwidth suppression mode,the congestion time allowable bandwidth 122 is assumed as the bandwidthof each of ONU 2, ONU 3, and ONU 6 that are transmitting upstream framesto the congestion port while the bandwidths of other ONUs that aretransmitting upstream frames to normal ports increase more than in thenormal mode. In FIG. 10, it is assumed that the OLT control part 11receives a congestion reset notice from the GW 50 while the ONU 7 istransmitting an upstream frame F27. In this embodiment, transientperiods T1 and T2 are generated before and after an optimal bandwidthcontrol period T2 in the bandwidth suppression mode.

FIG. 11 is a flowchart of downstream PON frame generation processings210 executed by the downstream frame generation part 21 in basic framecycles of ΔTF (in cycles of 125 microseconds).

The downstream frame generation part 21 generates a Psync field 611, aframe information field 612 (step 211), a bandwidth information field613 (step 212), and an ONU control information field 614 (step 213) ofthe GTC frame header (PCBd) shown in FIG. 2, then generates a GTCpayload 62 in which a user frame read from the downstream data buffer 20and a control frame generated by the OLT control part 11 are mapped inthe GEM frame format (step 214).

FIG. 12 is an example of a detailed flowchart of the bandwidthinformation field generation processings (step 212). The allocatedbandwidth 124 set by the OLT control part 11 in the bandwidth controltable 120 indicates an upstream bandwidth of each ONU in the ΔTF×Nperiod. The downstream frame generation part 21 divides an allocatedbandwidth 124 indicated in the bandwidth control table 120 into N×PONframes to be notified to every ONU 30 indicated with the bandwidthcontrol ID 121 in the bandwidth control table 120.

In FIG. 12, the parameter i indicates the basic frame cycle ΔTF in aperiod ΔTD shown in FIG. 10 and the parameter j indicates a position ofa table entry in the bandwidth control table 120. MAX indicates thenumber of basic frame cycles N included in a period ΔTD.

The downstream frame generation part 21 compares the value of theparameter i with “MAX+1” (step 220). If i=MAX+1 is satisfied, that is,if a new period ΔTD is set, the downstream frame generation part 21 sets“1” in the values of the parameters i and j (step 221) and initializesthe total value of the bandwidths to be allocated to ONUs (usable BW) inthe downstream PON frame (the first basic frame cycle) generated thistime (step 222).

The downstream frame generation part 21 then compares the parameter jwith the number of table entries in the bandwidth control table 120(step 223). If the parameter j is over the number of table entries, thedownstream frame generation part 21 exits the processing. If theparameter j is not over the number of table entries, the downstreamframe generation part 21 reads the j-th table entry from the bandwidthcontrol table 120 to check the congestion flag 126 (step 225). If “1” isset in the congestion flag 126, the downstream frame generation part 21sets the value of the congestion time allowable bandwidth 122 indicatedby the j-th table entry in the variable BW(j) (step 226). If “0” is setin the congestion flag 126, the downstream frame generation part 21 setsthe value of the allocated bandwidth 124 indicated by the j-th tableentry in the variable BW(j) (step 227).

After that, the downstream frame generation part 21 checks thecarry-over bandwidth 127 of the j-th table entry (step 228). If “0” isset in the carry-over bandwidth 127, the downstream frame generationpart 21 compares the usable BW with BW(j) (step 231). If “0” is not setin the carry-over bandwidth 127, the downstream frame generation part 21sets the value of the carry-over bandwidth 127 in BW(j) (step 229) andclears the carry-over bandwidth 127 of the j-th table entry (step 230),then compares the usable BW with BW(j) (step 231).

Here, the carry-over bandwidth means a bandwidth notified to each ONU inthe next basic frame period when a bandwidth (congestion time allowablebandwidth or allocated bandwidth) specified by the j-th table entry isover two basic frame cycles as shown in FIG. 10. In an actual case,however, it is possible to notify a bandwidth specified by the j-thtable entry in one basic frame cycle first, then subtract the carry-overbandwidth from the usable BW in the next basic frame cycle.

If the usable BW is over BW(j), the downstream frame generation part 21calculates both transmission starting time and transmission ending timeon the basis of the BW(j) (step 234). If the usable BW is under BW(j),the downstream frame generation part 21 sets the usable BW in BW(j)(step 232) and records the insufficient bandwidth in the j-th tableentry as a carry-over bandwidth (step 233), then calculates bothtransmission starting time and transmission ending time on the basis ofthe BW(j) (step 234).

After that, the downstream frame generation part 21 sets bandwidthcontrol information that includes both transmission starting time andtransmission ending time together with a bandwidth control ID indicatedby the j-th table entry in a downstream PON frame (step 235), thensubtracts the value of BW(j) from the usable BW (step 236) and comparesthe usable BW value with the predetermined bandwidth minimum value BWmin(step 237). If the usable BW value is over BWmin, the downstream framegeneration part 21 increases the parameter j value (step 238) andreturns the control sequence to step 223. If the usable BW value isunder BWmin, the downstream frame generation part 21 increases theparameter i value (step 239) and exits the processing.

FIG. 13 is a diagram for describing a congestion control sequenceaccording to the present invention, to be executed among the GW 50, OLT10, and ONU 30.

The GW 50 monitors the queue length QL of each output port (output lineinterface). If the queue length QL exceeds the threshold TH1, the GW 50issues a back pressure control frame (congestion occurrence notice)(congestion occurrence detection 401). If the queue length QL is underthe threshold value TH2, the GW 50 issues a back pressure control frame(congestion reset notice) that specifies a congestion port (congestionreset detection 404). The back pressure control frame (congestionoccurrence/reset notice) is transmitted to every OLT connected to the GW50.

The OLT 10 transmits downstream PON frames DF1, DF2, DF3, DF4, . . . inthe basic frame cycles ΔTF (SQ01, SQ02, SQ04, and SQ05). While theoutput port of the GW 50 is normal in state, the OLT 10 notifies eachONU of a bandwidth allocated in the normal mode. The ONU 30-k connectedto the OLT 10, when receiving a downstream PON frame DF2 includingbandwidth control information addressed to itself, starts transmissionof an upstream frame UFi at a proper timing (ΔTS) obtained by adding anequivalent delay to the transmission starting time indicated by thebandwidth control information (SQ03).

The OLT 10, when receiving a congestion occurrence notice from the GW50, sets “1” in the congestion flag corresponding to the notified GWoutput port number in the congestion control table 130 and in thebandwidth control table 120 respectively (step 402). After “1” is set inthe congestion flag, a period until the OLT 10 allocates a bandwidth inthe band allocation cycle ΔTD (step 403) and the bandwidth control table120 is updated becomes a transient period T1 shown in FIG. 10.

If the bandwidth control table 120 is updated while the GW 50 is in acongestion state, the bandwidth control by the OLT 10 becomes theoptimal control period T2. The downstream PON frames (SQ11, SQ12, andSQ14) transmitted in the optimal control period T2 notify each ONU thatuses a GW normal port of an extended allocation bandwidth (E-BWs) andnotify each ONU that uses a congestion port of a congestion timeallowable bandwidth (C-BWs). Consequently, if the ONU 30-k uses acongestion port, the ONU 30-k comes to transmit upstream frames UFi+1(SQ13) in the congestion time allowable bandwidth.

The OLT 10, when receiving a congestion reset notice from the GW 50,records a congestion reset port number (step 405), then sets “0” in thecongestion flag in the next allocation processing in the bandwidthallocation cycle ΔTD (step 406). Consequently, a period between when theGW 50 issues a congestion reset notice and when the OLT 10 allocates abandwidth (step 406) becomes a transient period T3 in which a downstreamPON frame (SQ15) notifies each ONU of an extended allocation bandwidthor congestion time allowable bandwidth.

If the OLT 10 that has received a congestion reset notice updates thebandwidth control table 120 once, the OLT 10 enters the normal mode inwhich the LOT 10 notifies each ONU with a downstream PON frame (SQ21 andSQ22) of a bandwidth (U-BWs) to be allocated normally. Consequently, theONU 30-k for which the transmission bandwidth has been suppressed comesto be able to transmit upstream frames UFi+2 in the ordinary allocatedbandwidth (SQ23).

FIG. 14 is a graph for describing a relationship between bandwidthcontrolling by the OLT 10 and a temporal change of the amount of theaccumulated data (transmission queue length) in an output frame buffer518 observed by an output frame buffer 519 of the GW 50. TH1 indicatesthe first threshold value for detecting congestion occurrence and TH2indicates the second threshold value for detecting congestion reset. ΔTDindicates a period of bandwidth calculation by the OLT 10.

The GW management part 53 reads a transmission queue length QL from theoutput frame buffer 519 of the each output port (output line interfacepart) 51B of the GW 50 in a predetermined cycle, for example, in thebasic frame cycle ΔTF and instructs the back pressure control framegeneration part 521 to issue a congestion occurrence notification framewhen the queue length QL exceeds the first threshold value TH1 andinstructs the back pressure control frame generation part 521 to issue acongestion reset notification frame when the congestion occurred outputport queue length QL falls below the second threshold value TH2.Consequently, the GW 50 assumes a period between congestion occurrenceand congestion reset as a congestion period.

The first threshold value TH1 is set at a value smaller than thecapacity of the output frame buffer 518. Thus the congestion occurrencenotification frame is issued before the output frame buffer becomesfull. And the capacity of the output frame buffer 518 remains stillenough to absorb an increase of the queue length until each OLT 10suppresses the bandwidth in response to a congestion occurrence notice.Receiving a congestion occurrence notice from the GW 50, the OLT controlpart 11 controls the bandwidth allocated to each ONU in the bandwidthsuppression mode consisting of the above described transient periods T1and T3, as well as the optimal control period T3.

FIG. 15 is an explanatory diagram for describing the bandwidthsuppression mode periods 151 and 152 in an OLT 10 and the temporalchanges of both transmission queue length 153 and allocated bandwidth154 in an ONU that is transmitting frames that have passed a congestionoccurred GW port (hereunder, to be referred to as a congestion pathONU).

FIG. 16 is an explanatory graph for describing the bandwidth suppressionmode periods 151 and 152 in an OLT 10 and the temporal changes of bothtransmission queue length 153 and allocated bandwidth 155 in an ONU thatis transmitting frames that have passed a normal GW port (hereunder, tobe referred to as a normal path ONU).

In order to make it easier to understand the effect of the bandwidthcontrolling according to the present invention, assume here that thetransmission queue length is changed in the same way in both normal pathONU and congestion path ONU and only the change of an allocatedbandwidth is checked between those ONUs. The bandwidth controlling inthe OLT control part 11 is affected on each ONU just with a delay of thebasic frame cycle ΔTF. In any period other than the periods 151 and 152in the bandwidth suppression mode, an allocated bandwidth BW(j) isnotified to each ONU according to the transmission queue length 153calculated as a bandwidth in the normal mode.

A period between when the OLT control part 11 enters a bandwidthsuppression mode period 151 between times t1 and t2 and when the nextbandwidth is allocated becomes a transient period T1. If the nextallocated bandwidth is notified to a congestion path ONU in thetransient period T1, the congestion path ONU bandwidth is suppressed tothe congestion time allowable bandwidth Bcon within the transient periodT1 as shown in FIG. 15. If the OLT control part 11 allocates a bandwidthjust before the time t2, the bandwidth suppression mode enters theoptimal control period T2. In this embodiment, the bandwidth allocatedto a congestion path ONU is kept at the congestion time allowablebandwidth Bcon and this state is continued even after the bandwidthsuppression mode goes into the transient period T3 after the GW outputport goes out of the congestion state. The congestion path ONU bandwidthreturns to a bandwidth allocated according to the transmission queuelength SQ when the bandwidth controlling of the OLT 10 goes into thenormal mode.

According to the present invention, because only the bandwidth allocatedto a congestion path ONU is deleted when the OLT 10 enters the bandwidthsuppression mode, each normal path ONU is allocated a bandwidth asusually in a transient period T1 of the bandwidth suppression mode asshown in FIG. 16. When the bandwidth suppression mode of the OLT controlpart 11 enters the optimal control period T2, a surplus bandwidthgenerated in the PON section due to the deletion of the congestion pathONU is distributed to each of plural normal path ONUs according to thetransmission queue length. Consequently, as shown in FIG. 16, in theoptimal control period T2, the bandwidth to be allocated to each normalpath ONU increases. However, because a bandwidth for each normal pathONU is allocated just like in the normal mode according to thetransmission queue length, the allocated bandwidth is reduced inproportion to the reduction of the transmission queue length SQ as shownat the time t8.

In the embodiment described above, the OLT control part 11 keeps thecongestion path ONU bandwidth at the congestion time allowable bandwidthBcon in the bandwidth suppression mode period. However, it is alsopossible to allocate a bandwidth to the congestion path ONU according tothe source queue length within the congestion time allowable bandwidth.In the same way, it is also possible to allocate a bandwidth to eachnormal path ONU according to the source queue length within the maximumcontrol bandwidth 123 indicated in the bandwidth control table 120.

In this embodiment, the congestion time allowable bandwidth of each ONUis given as a fixed value. However, the allowable bandwidth may be adynamic value obtained by multiplying the value of a bandwidth allocatedto each ONU before the congestion occurs by a predetermined reductionrate.

Although a back pressure control frame generation part 521 is providedfor each output line interface part 51B of the GW 50 and the backpressure control frame generation part 521 generates both congestionoccurrence notification frame and congestion reset notification frameaccording to the commands from the GW management part 53 in the aboveembodiment, the GW management part 53 may generate the back pressurecontrol frames (congestion occurrence notification frame and congestionreset notification frame).

In such a case, a back pressure control frame buffer is provided foreach output line interface part 51B instead of the back pressure controlframe generation part 521 and for example, an OLT connection portnumber, a connection port MAC address, an OLT MAC address correspondingto a GW output port number respectively are recorded in the managementinformation table 54 beforehand. The GW management part 53 thus refersto the management information table 54 when detecting congestionoccurrence in an output port Pi to generate a back pressure controlframe and output the control frame to the back pressure control framebuffer of the output line interface part 51B corresponding to the OLTconnection port number. The output frame reading part 520 is allowed toread and transmit frames according to the priority given to the backpressure control frame buffer over the output frame buffer 518.

Second Embodiment

FIG. 17 is another embodiment of the network to which the presentinvention applies.

In this embodiment, GWs 500-1 and 500-2 of a carrier network areprovided with a line interface (PON interface) having OLT functionsrespectively and houses PON optical fibers 106 (106-1-1 to 106-2-n)directly.

FIG. 18 is a configuration of the GW 500-1.

The GW 500-1 consists of plural network interface parts 510 (510-1 to510-q), a frame relay part 52 for connecting those interface parts 510to each other, a GW management part 53, and a management informationtable 54. The network interface part 51-1 for housing the PON opticalfiber 106 is provided with a PON interface (IF) 5110 instead of theEthernet IF 511 shown in FIG. 8. The network interface part for housingthe carrier network lines 101 (101-1 and 101-2) is provided with anEthernet IF 511 shown in FIG. 8.

The PON interface (IF) 5110 is the same as that of the OLT 10 shown inFIG. 4 except for that the Ethernet IF 14, the upstream frametransmission part 18, and the frame analysis part 19 are excluded fromthe configuration. And the OLT control part 11 is connected to the GWmanagement part 53 in this embodiment.

According to this embodiment, because the OLT control part 11 canreceive both congestion occurrence notice and congestion reset noticefrom the GW management part 53 directly, it is possible to omit themanagement of the destination MAC address required for generating a backpressure control frame shown in FIG. 3. Furthermore, according to thisembodiment, because the OLT control part 11 can quickly reduce abandwidth allocated to a congestion path ONU when congestion occurs inan output port in a GW, it is possible to quicken avoiding of congestionwith a back pressure for the congestion path ONU, thereby the GW outputframe buffer memory can be reduced in capacity.

Although an example of application to the G-PON has been described inthe first and second embodiments, the bandwidth controlling of thepresent invention may also apply to other types of PON, for example, tothe GE-PON.

1. A passive optical network (PON) system, comprising: an optical lineterminal (OLT); a plurality of optical network units (ONUs) connected tothe OLT through a passive optical network respectively; a gatewayconnected to the OLT through a communication line; wherein the OLTincludes: correspondence information between an output port number ofthe GW and an identifier (ID) of an optical network unit (ONU) that usesa GW output port identified with the output port number; an OLT controlpart that allocates a bandwidth distributed to each of the plurality ofONUs dynamically as the next upstream frame transmission bandwidth; anda downstream frame generation part that generates a downstream frameincluding bandwidth control information indicating an upstream frametransmission bandwidth for each ONU according to a frame transmissionbandwidth allocated by the OLT control part, wherein the OLT controlpart, when receiving a congestion occurrence notice indicating a numberof the output port in which congestion occurred from the GW, identifiesthe ONU corresponding to the congestion output port number in thecorrespondence information and allocates the upstream frame transmissionbandwidth to the identified ONU which is less than the bandwidthallocated to the identified ONU before the congestion of the output portoccurred.
 2. A PON system according to claim 1, wherein the OLT controlpart, when receiving a congestion occurrence notice, allocates theupstream frame transmission bandwidth to one of the ONUs other than theidentified ONU which is more than the bandwidth allocated to the one ofthe ONUs other than the identified ONU before the congestion of theoutput port occurred.
 3. A PON system according to claim 1, wherein theOLT control part, when the congestion of the GW resolves, receives acongestion rest notice indicating a number of the output port of thecongestion reset output port from the GW, identifies the identifier ofan ONU corresponding to the congestion reset output port number based onthe number of the output port and the correspondence information, andallocates the upstream frame transmission bandwidth to the identifiedONU which is more than the bandwidth allocated to the identified ONU inthe situation where the congestion of the output port occurred.
 4. A PONsystem according to claim 1, wherein the OLT control part has acongestion flag which indicates whether or not the congestion occurs atthe output port of GW corresponding to the ONU identifier, when one ofthe plurality of ONUs flags indicates the congestion occurs, allocatesthe upstream frame transmission bandwidth to the ONU whose congestionflag indicates the congestion occurs less than the bandwidth allocatedto the ONU before the congestion of the output port occurred andallocates the upstream frame transmission bandwidth to the ONU whosecongestion flag indicates the congestion does not occur which is morethan the bandwidth allocated to the ONU before the congestion of theoutput port occurred.
 5. A PON system according to claim 1, wherein theOLT control part receives a congestion occurrence notice from the GW,the OLT control part changes the congestion flag corresponding to theONU ID identified the output port number including the congestionoccurrence notice to the status that indicates the congestion hasoccurred, when the OLT control part receives a congestion reset noticefrom the GW, the OLT control part changes the congestion flagcorresponding to the ONU ID identified the output port number includingthe congestion reset notice to the status that indicates the congestionhas not occurred.
 6. A PON system according to claim 1, wherein the OLThas a fixed bandwidth to be allocated to the ONU using the output portin which the congestion has occurred and allocates the fixed bandwidthto the ONU using the output port in which the congestion has occurred.7. A PON system according to claim 1, wherein the GW sends a congestionoccurrence notice to the OLT when an upstream transmission bandwidth ofan output port of the GW exceeds a predetermined threshold.
 8. A PONsystem according to claim 1, wherein the GW sends a congestion resetnotice to the OLT when an upstream transmission bandwidth of an outputport of the GW in which congestion has occurred falls below apredetermined threshold.
 9. A PON system according to claim 2, whereinthe OLT control part processes a bandwidth allocating processing to theplurality of the ONUs with a cycle which is longer than a cycle whichgenerates a downstream frame at the downstream frame generation part andallocates the upstream transmission bandwidth which is more than thebandwidth allocated before receiving the congestion occurrence notice toONUs other than the identified ONU by the bandwidth allocatingprocessing which processes after receiving the congestion occurrencenotice.
 10. A PON system according to claim 4, wherein the OLT controlpart processes a bandwidth allocating processing to the plurality of theONUs with a cycle which is longer than a cycle which generates adownstream frame at the downstream frame generation part, upon receivingthe congestion reset notice, identifies the ONU ID corresponding to theoutput port included in the congestion reset notice based on thecorrespondence information, and changes the congestion flagcorresponding to the identified ONU to the status that indicates thecongestion has not occurred upon the next processing.
 11. An opticalline terminal (OLT) connected to a plurality of optical network units(ONUs) through a passive optical network (PON) respectively, the OLTcomprising: correspondence information between an output port number ofa gateway (GW) connected to the OLT through a communication line and anidentifier (ID) of an optical network unit (ONU) that uses a GW outputport identified with the output port number; an OLT control part thatallocates a bandwidth distributed to each of the ONUs dynamically as thenext upstream frame transmission bandwidth; and a downstream framegeneration part that generates a downstream frame including bandwidthcontrol information indicating an upstream frame transmission bandwidthfor each ONU according to a frame transmission bandwidth allocated bythe OLT control part, wherein the OLT control part, when receiving acongestion occurrence notice indicating a number of the output port inwhich congestion occurred from the GW, identifies the ONU identifiercorresponding to the congestion output port number in the correspondenceinformation and allocates the upstream frame transmission bandwidth tothe identified ONU less than the bandwidth allocated to the identifiedONU before the congestion of the output port occurred.
 12. An opticalline terminal (OLT) according to claim 11, wherein the OLT control part,when receiving a congestion occurrence notice, allocates the upstreamframe transmission bandwidth to one of the ONUs other than theidentified ONU which is more than the bandwidth allocated to the one ofthe ONUs other than the identified ONU before the congestion of theoutput port occurred.
 13. An optical line terminal (OLT) according toclaim 11, wherein the OLT control part, when the congestion of the GWresolves, receives a congestion rest notice indicating a number of theoutput port of the congestion reset output port from the GW, identifiesthe identifier of an ONU corresponding to the congestion reset outputport number based on the number of the output port and thecorrespondence information, and allocates the upstream frametransmission bandwidth to the identified ONU which is more than thebandwidth allocated to the identified ONU in the situation where thecongestion of the output port occurred.
 14. An optical line terminal(OLT) according to claim 11, wherein the OLT control part has acongestion flag which indicates whether or not the congestion occurs atthe output port of GW corresponding to the ONU identifier, when one ofthe plurality of ONUs flags indicates the congestion occurs, allocatesthe upstream frame transmission bandwidth to the ONU whose congestionflag indicates the congestion occurs less than the bandwidth allocatedto the ONU before the congestion of the output port occurred andallocates the upstream frame transmission bandwidth to the ONU whosecongestion flag indicates the congestion does not occur which is morethan the bandwidth allocated to the ONU before the congestion of theoutput port occurred.
 15. An optical line terminal (OLT) according toclaim 11, wherein the OLT control part receives a congestion occurrencenotice from the GW, the OLT control part changes the congestion flagcorresponding to the ONU ID identified the output port number includingthe congestion occurrence notice to the status that indicates thecongestion has occurred, when the OLT control part receives a congestionreset notice from the GW, the OLT control part changes the congestionflag corresponding to the ONU ID identified the output port numberincluding the congestion reset notice to the status that indicates thecongestion has not occurred.
 16. An optical line terminal (OLT)according to claim 11, wherein the OLT has a fixed bandwidth to beallocated to the ONU using the output port in which the congestion hasoccurred and allocates the fixed bandwidth to the ONU using the outputport in which the congestion has occurred.
 17. An optical line terminal(OLT) according to claim 11, wherein the GW sends a congestionoccurrence notice to the OLT when an upstream transmission bandwidth ofan output port of the GW exceeds a predetermined threshold.
 18. Anoptical line terminal (OLT) according to claim 11, wherein the GW sendsa congestion reset notice to the OLT when an upstream transmissionbandwidth of an output port of the GW in which congestion has occurredfalls below a predetermined threshold.
 19. An optical line terminal(OLT) according to claim 12, wherein the OLT control part processes abandwidth allocating processing to the plurality of the ONUs with acycle which is longer than a cycle which generates a downstream frame atthe downstream frame generation part and allocates the upstreamtransmission bandwidth which is more than the bandwidth allocated beforereceiving the congestion occurrence notice to ONUs other than theidentified ONU by the bandwidth allocating processing which processesafter receiving the congestion occurrence notice.
 20. An optical lineterminal (OLT) according to claim 14, wherein the OLT control partprocesses a bandwidth allocating processing to the plurality of the ONUswith a cycle which is longer than a cycle which generates a downstreamframe at the downstream frame generation part, upon receiving thecongestion reset notice, identifies the ONU ID corresponding to theoutput port included in the congestion reset notice based on thecorrespondence information, and changes the congestion flagcorresponding to the identified ONU to the status that indicates thecongestion has not occurred upon the next processing.
 21. A passiveoptical network (PON) system, comprising: a gateway (GW) having a PONinterface; a plurality of optical network units (ONUs) connected to theGW through a PON respectively; wherein the GW includes: correspondenceinformation between an output port number of the GW and an identifier(ID) of an optical network unit (ONU) that uses a GW output portidentified with the output port number; a PON interface control partthat allocates a bandwidth distributed to each of the plurality of ONUsdynamically as the next upstream frame transmission bandwidth; and adownstream frame generation part that generates a downstream frameincluding bandwidth control information indicating an upstream frametransmission bandwidth for each ONU according to a frame transmissionbandwidth allocated by the PON interface control part, wherein the PONinterface control part, when receiving a congestion occurrence noticeindicating a number of the output port in which congestion occurred fromthe GW, identifies the ONU identifier corresponding to the congestionoutput port number in the correspondence information and allocates theupstream frame transmission bandwidth to the identified ONU less thanthe bandwidth allocated to the identified ONU before the congestion ofthe output port occurred.
 22. A PON system according to claim 21,wherein the PON interface control part, when receiving a congestionoccurrence notice, allocates the upstream frame transmission bandwidthto one of the ONUs other than the identified ONU which is more than thebandwidth allocated to the one of the ONUs other than the identified ONUbefore the congestion of the output port occurred.
 23. A PON systemaccording to claim 21, wherein the PON interface control part, when thecongestion of the GW resolves, receives a congestion rest noticeindicating a number of the output port of the congestion reset outputport from the GW, identifies the identifier of an ONU corresponding tothe congestion reset output port number based on the number of theoutput port and the correspondence information, and allocates theupstream frame transmission bandwidth to the identified ONU which ismore than the bandwidth allocated to the identified ONU in the situationwhere the congestion of the output port occurred.
 24. A PON systemaccording to claim 21, wherein the PON interface control part has acongestion flag which indicates whether or not the congestion occurs atthe output port of GW corresponding to the ONU identifier, when one ofthe plurality of ONUs flags indicates the congestion occurs, allocatesthe upstream frame transmission bandwidth to the ONU whose congestionflag indicates the congestion occurs less than the bandwidth allocatedto the ONU before the congestion of the output port occurred andallocates the upstream frame transmission bandwidth to the ONU whosecongestion flag indicates the congestion does not occur which is morethan the bandwidth allocated to the ONU before the congestion of theoutput port occurred.
 25. A PON system according to claim 22, whereinthe PON interface control part processes a bandwidth allocatingprocessing to the plurality of the ONUs with a cycle which is longerthan a cycle which generates a downstream frame at the downstream framegeneration part and allocates the upstream transmission bandwidth whichis more than the bandwidth allocated before receiving the congestionoccurrence notice to ONUs other than the identified ONU by the bandwidthallocating processing which processes after receiving the congestionoccurrence notice.
 26. A PON system according to claim 24, the PONinterface control part processes a bandwidth allocating processing tothe plurality of the ONUs with a cycle which is longer than a cyclewhich generates a downstream frame at the downstream frame generationpart, upon receiving the congestion reset notice, identifies the ONU IDcorresponding to the output port included in the congestion reset noticebased on the correspondence information, and changes the congestion flagcorresponding to the identified ONU to the status that indicates thecongestion has not occurred upon the next processing.
 27. A gateway(GW), the GW comprising: a passive optical network (PON) interfaceconnected to a plurality of optical network units (ONUs) through a PONrespectively; correspondence information between an output port numberof the GW and an identifier (ID) of an optical network unit (ONU) thatuses a GW output port identified with the output port number; a PONinterface control part that allocates a bandwidth distributed to each ofthe ONUs dynamically as the next upstream frame transmission bandwidth;and a downstream frame generation part that generates a downstream frameincluding bandwidth control information indicating an upstream frametransmission bandwidth for each ONU according to a frame transmissionbandwidth allocated by the PON interface control part, wherein the PONinterface control part, when receiving a congestion occurrence noticeindicating a number of the output port in which congestion occurred fromthe GW, identifies the ONU identifier corresponding to the congestionoutput port number in the correspondence information and allocates theupstream frame transmission bandwidth to the identified ONU which isless than the bandwidth allocated to the identified ONU before thecongestion of the output port occurred.
 28. A gateway (GW) according toclaim 27, wherein the OLT control part, when receiving a congestionoccurrence notice, allocates the upstream frame transmission bandwidthto one of the ONUs other than the identified ONU which is more than thebandwidth allocated to the one of the ONUs other than the identified ONUbefore the congestion of the output port occurred.
 29. A gateway (GW)according to claim 27, wherein the PON interface control part, when thecongestion of the GW resolves, receives a congestion rest noticeindicating a number of the output port of the congestion reset outputport from the GW, identifies the identifier of an ONU corresponding tothe congestion reset output port number based on the number of theoutput port and the correspondence information, and allocates theupstream frame transmission bandwidth to the identified ONU which ismore than the bandwidth allocated to the identified ONU in the situationwhere the congestion of the output port occurred.
 30. A gateway (GW)according to claim 27, wherein the PON interface control part has acongestion flag which indicates whether or not the congestion occurs atthe output port of GW corresponding to the ONU identifier, when one ofthe plurality of ONUs flags indicates the congestion occurs, allocatesthe upstream frame transmission bandwidth to the ONU whose congestionflag indicates the congestion occurs less than the bandwidth allocatedto the ONU before the congestion of the output port occurred andallocates the upstream frame transmission bandwidth to the ONU whosecongestion flag indicates the congestion does not occur which is morethan the bandwidth allocated to the ONU before the congestion of theoutput port occurred.
 31. A gateway (GW) according to claim 28, whereinthe PON interface control part processes a bandwidth allocatingprocessing to the plurality of the ONUs with a cycle which is longerthan a cycle which generates a downstream frame at the downstream framegeneration part and allocates the upstream transmission bandwidth whichis more than the bandwidth allocated before receiving the congestionoccurrence notice to ONUs other than the identified ONU by the bandwidthallocating processing which processes after receiving the congestionoccurrence notice.
 32. A gateway (GW) according to claim 30, the PONinterface control part processes a bandwidth allocating processing tothe plurality of the ONUs with a cycle which is longer than a cyclewhich generates a downstream frame at the downstream frame generationpart, upon receiving the congestion reset notice, identifies the ONU IDcorresponding to the output port included in the congestion reset noticebased on the correspondence information, and changes the congestion flagcorresponding to the identified ONU to the status that indicates thecongestion has not occurred upon the next processing.